Go with the factory mag. The ad in the link above is a good price. You can get them on Ebay as well for $40-50.
I messed around with aftermarket mags and got them to work fairly well, but other than range fun, I would never depend on them. Get a good factory mag and you'll never worry about it.
The next step is to get good stripper clips. The NcStar ones are absolute crap, most won't fit due to poor machining, and the ones the will either retain the rounds and feed poorly, or feed well and have bullets fall out in storage -- never both. Get some surplus ammo clips, either Yugo, Chinese, or Czech. The easiest and best ones to get recently are those marked "BXN". Search Ebay. Expect to pay between $1.00 - 1.50 per clip, shipped. It's very much worth the money. Here are a couple of good vendors that I've bought from:
http://www.ebay.com/itm/271754398314
http://www.ebay.com/itm/40-SKS-Sks-10rd-Stripper-Clips-Czech-BXNs-Excellent-/221697051819?pt=LH_DefaultDomain_0&hash=item339e2c8cab
Just make sure that they're Czech-made, not just "made for VZ58, SKS".
The difference between good surplus clips and the garbage NcStar makes is night and day.